An essential goal of the nervous system is to maintain orientation and equilibrium relative to the outside world. Normal performance requires that we accurately localize ourselves within our surrounding, distinguish self-movement from that of environment elements, and control movement through a complex world. These tasks are accomplished by neural systems that coordinate and integrate multiple motor behaviors with the aid of feedback from multiple sensory modalities. Vision and audition (the exteroceptive position of the body and its parts, and vestibular inputs yield head motion and orientation information. These sensory inputs are then processed by the brain, which generates the motor behaviors required to perform natural activities. Examples include coordinated eye and head movements that acquire and hold fixation on targets of interest, guided hand movements that allow us to accurately grasp and manipulate objects, and maintenance of postural and gait stability required for ambulation. How these crucial and ubiquitous functions work and maintain coordination, and how they degenerate with natural aging, constitute the subject of the proposed research. This endeavor is motivated by two public health concerns: aging is accompanied by a progressive increase in the clinical prevalence of dysequilibrium, and in the risk and consequences of falls. Both phenomena constitute failures of spatial orientation and equilibrium control. We will study physiological and psychophysical parameters that underlie how the brain utilizes vestibular, visual, proprioceptive, and auditory inputs to control eye-head-hand coordination and postural stability. How these functions deteriorate with aging will be determined. Further, we will quantify adaptive capabilities that maintain accurate calibration between sensory modalities and between exteroceptive input and motor output (e.g. accurate ocular fixation or grasping), and will elucidate potential age-dependent limitations in these adaptive capabilities. State of the art techniques will be employed to isolate and combine specific sensory cues relevant to spatial localization and to the motor acquisition of external targets, as well as to the control of postural stability. We hypothesize that specific deficits in sensory interactions, sensorimotor integration, and adaptive plasticity occur with aging. Further, a thorough understanding of these functions is required in order to develop specific and effective interventions that might correct, or rehabilitate, potential age-dependent or disease-induced deficits.